The invention relates to a brake booster with a variable boost ratio. The invention is more particularly applicable to brake boosters for motor vehicles.
A brake booster like the one depicted in FIG. 1 essentially comprises, in a sealed casing 10, two chambers (the front chamber 10av and the rear chamber 10ar) which are separated by a moving membrane 14.
A control rod 20 can move towards the front of the vehicle (in the direction of arrow F1 in FIG. 1) when the driver of the vehicle depresses the brake pedal. This movement of the control rod 20 is transmitted to a slide valve plunger 21 which actuates boost means. As is known in the art, these boost means generally comprise a three-way valve, actuation of which makes it possible either to place the front chamber 10av of the booster in communication with the rear chamber 10ar, or to interrupt this communication, or to place the rear chamber in communication with the surrounding atmosphere.
The front chamber 10av is normally under vacuum. At rest (when there is no brake command), the two, front and rear, chambers are in communication with one another and the same pressure is obtained in each of the two chambers.
When the driver depresses the brake pedal, the control rod 20 is moved forward (to the left in the figure). The slide valve plunger 21 is moved forward and causes the three-way valve to operate. This valve isolates the two, front and rear, chambers and places the rear chamber in communication with atmospheric pressure.
Because of the difference in pressure between the two chambers, a boost force is applied to the moving membrane 14 separating the two chambers. This moving membrane 14 is associated with a skirt 15 made of a rigid material. Under the effect of the boost force, the moving membrane tends to move forward and carries the skirt 15 along with it.
The skirt 15 itself carries along the piston 22 which therefore moves forward (in the direction of the arrow F1), acting on a push rod 30 which serves to actuate a piston of the master cylinder 61 of the braking circuit.
In the conventional way, in a booster design such as this, the slide valve plunger 21 slides in a bore of the hub of the piston 22 and forms a thrust member acting on a reaction disk 23 which transmits force to the push rod 30.
This reaction disk 23 rests against the front face of the plunger and against the front face of the piston surrounding the plunger.
In a system such as this, the brake booster has a fixed boost ratio. However, under certain operating circumstances, for example in the event of emergency braking, it may be advantageous to provide a boost force as early as possible.
This is why, in some systems, there is, between the control rod 20 and the reaction disk 23, a sensor feeler which slides in a cylinder able to move axially inside the piston. FIGS. 2a and 2b depict a system such as this which is described, for example, in Patent EP 1 322 510.
In this system, there is therefore a sensor feeler 31 able to move axially inside a cylinder 34 which is able to move axially inside the piston 22. The spring 32 tends to push the cylinder 34 to the left and to bring the end 35 of the sensor feeler 31 closer to the left-hand end of the cylinder 34. In that system, the force of the control rod 20 is exerted via the slide valve plunger 21 on the sensor feeler 31.
The invention relates to an improvement to this type of device that has the advantage of being easier to manufacture and of reducing the time taken to assemble the booster.
The invention therefore relates to a sensor feeler device for a brake booster, comprising:
a pneumatic control device controlled by a three-way valve,
a slide valve plunger providing control of the pneumatic control device,
a control rod intended to apply an axial movement force to the slide valve plunger,
a piston able to move axially under the control of the pneumatic control device,
a push rod receiving axial movement forces from said moving piston and from the slide valve plunger and forwarding them to a brake master cylinder,
a reaction disk positioned between the control rod and the push rod,
a sensor feeler device able to move axially and situated between the reaction disk and the control rod.
According to the invention, the sensor feeler device comprises:
an axially moveable cylinder the movement of which can be controlled by the control rod,
a sensor feeler piston able to move axially in said moving cylinder,
a spring that tends to push the sensor feeler piston forward and keep its front face substantially in the same plane as the front face of the moving cylinder.
According to one advantageous embodiment of the invention, the moving cylinder comprises a cavity capable of accepting, such that it can slide axially, the moving sensor feeler piston. The moving cylinder comprises a shoulder that forms a stop for a thrust piece of the sensor feeler piston and limits the forward movement of the sensor feeler piston.
In addition, the moving cylinder advantageously comprises a rear part. Said spring rests, on the one hand, against this rear part and, on the other hand, against the sensor feeler piston.
According to one embodiment, said rear part is welded to the moving cylinder.
According to another form of embodiment, the said rear part and the moving cylinder are made as one piece.
Provision may also be made for the side wall of the moving cylinder to comprise an aperture to allow the thrust piece of the sensor feeler piston to slide.
Advantageously, provision will also be made for the rear part to comprise a first axial boss for centering the spring.
According to an alternative form of embodiment, the sensor feeler piston has a second axial boss also for centering the spring.
To allow the plunger piston to be fitted into the moving cylinder, the axial length of the aperture and the distance separating the front face of the first boss from the shoulder of the moving cylinder are both greater than the total axial length of the plunger piston. In this embodiment, in which the plunger piston and the thrust piece are made in two parts, provision may be made for this distance simply to be greater than the thickness of the thrust piece of the plunger piston.
In this form of embodiment, said aperture is wide enough to allow the thrust piece of the plunger piston through.
According to an alternative form of embodiment of the invention, the height of the aperture is greater than the total axial length of the plunger piston, of its thrust piece and of its boss.
Provision may also be made for the plunger piston and its shoulder to be made in two parts.